This invention relates to an oscillation circuit and a technique effectively applicable to a voltage controlled oscillation circuit integrated with a power supply circuit or in particular to a technique effectively applicable to a voltage controlled oscillation circuit (hereinafter referred to as the VCO) for generating a radio-frequency local oscillation signal used for down-converting a receiving signal or up-converting a transmission signal.
A radio communication device such as a mobile phone uses a communication semiconductor integrated circuit (hereinafter referred to as the radio-frequency IC) for down-converting or up-converting the frequency by combining the receiving signal or the transmission signal with a radio-frequency local oscillation signal or modulating the transmission signal or demodulating the receiving signal. A radio-frequency IC having a built-in VCO for generating a radio-frequency local oscillation signal to be combined with the received signal or the transmission signal has been proposed.
Generally, the VCO has the frequency dependency in which the oscillation frequency changes with the source voltage. To obviate this problem, a circuit as shown in FIG. 1 is available in which a power supply circuit capable of generating a stable source voltage and a voltage controlled oscillation circuit are integrated with each other to suppress the source voltage variations. The circuit shown in FIG. 1 includes a power supply circuit 10 having a circuit 11 such as a band gap reference circuit for generating a constant voltage and a voltage regulator 12 for generating a voltage corresponding to the constant voltage, and an oscillation circuit 20 oscillated with the voltage from the power supply circuit as a source voltage.